Full Auto Malfunction PITA
I was trying to think of a good place to put this and figured it would get more exposure here. (Mods feel free to move it to NFA if needed).
The weapon in question is a Bushmaster 14.5" select-fire M4gery. Ammunition used was new production commercial Lake City M855 (no it is not pull down or reject ammo) according to the email from the Lake City-ATK rep.
Operator was on the range conducting BZO of carbine. Weapon was zeroed and no malfunctions were encountered. Operator switched into full-auto and that's when the fun began. It would fire random burst and then stop. After it was reported I went out to the range to observe first hand and test it myself. What I saw was that the weapon would stop firing, a live round was in the chamber (primer had been lightly struck) and the hammer was forward.
Having just inspected the weapon and gaged it I was rather dumbfounded as to what was going on. I assumed that the hammer was somehow tripping late and was following the bolt carrier forward however there was not enough energy to ignite the primer. Weapon went back to shop and was torn down. I didn't like the look of some of the parts so I replaced the entire fire control group, replaced the buffer spring (was still within spec) rechecked the BCG, replaced the gas rings and checked the gas tube, etc... and couldn't find anything wrong.
Went back out to range after completion of work and test fired again. Weapon malfunctioned as previous. Now I am scratching my head because I have changed everything that I can think of with no change in results.
Went back to the shop checked everything again-all was good. I had a sneaky suspicion as to what the problem was but I was in denial. I tested my theory by getting another carbine, removing the upper that had just had the barrel replaced, chamber reamed, gaged, cleaned, etc... and we swapped them over. Went back out to the range and conducted a test fire.
As I suspected the new upper was 100% and no malfunctions. The other upper on the different lower did the exact same thing as previous. Fired a few burst and then stopped. My final conclusion was the gas port in the barrel was eroded and it was undetected until the weapon was fired in full-auto. Unless I missed something there is no other logical explanation.
I would like to hear from Grant, Robb, and any others out there to see what their take is on this. Earlier today a second carbine did the same thing so I will be validating my results in the next day or so with another test.
If you don't mind an aside. . . .
Just wondering, but would it be possible to test for an eroded/enlarged gas port by measuring the velocity of the bullet? Would the larger volume of gas going through the port lower the velocity of the bullet relative to a properly sized port, or are we talking such a small amount of difference that you couldn't measure it given the inconsistency of the ammo?
I have very little technical knowledge of the platform, so I apologize if the question is misplaced and also for not offering any real input to the discussion. I was just wondering after reading through the thread.
Jason